If you live somewhere that gets snow and ice in the winter, you've likely enjoyed a “snow day" off from school when snow and ice made the roads too dangerous for driving.

As your city works to clear snow and ice from the roads, you may have noticed trucks spilling salt or sand on the road. That's no accident! It's all part of the plan.

In warm water, molecules move quickly and freely, making liquids fluid and able to adapt to any shape of container you pour them into. As water freezes, though, molecules begin to move more slowly. Eventually the molecules begin sticking together to form ice in a process we call freezing.

When you put salt on ice, liquid water molecules that have not yet frozen on the surface of the ice dissolve a bit of the salt, creating a saltwater solution called “brine." The brine prevents the liquid water molecules from freezing.

As more water molecules begin to speed up and break free from the ice, they turn into liquid and dissolve even more of the salt.

Why does this happen? Salt lowers the freezing point of water. Ice forms when water is cooled to 32° F (0° C). When you add salt to water, the freezing point drops.

If you add a 10-percent salt solution, the freezing point drops to 20° F. A 20-percent salt solution freezes at 2° F.

If you have ever sprinkled salt on your icy front porch steps, you already know you can watch this process in action. When you sprinkle salt on ice, you will notice that it begins to dissolve immediately around each grain of salt.

This melting continues to spread, until you are left with salt grains on the porch steps. With the ice gone, the salt also provides traction, making it safer for visitors to come to your front door.

Unfortunately, if temperatures dip below approximately 15° F, salt may not get the job done. Salt needs water to begin the melting process.

If all the water is frozen solid, it cannot get into the ice structure to start the melting. When this happens, workers often turn to sand instead.

Many people believe that sand and salt are used for the same purpose — to melt the ice. But they're wrong!

While workers use salt to melt ice, they rely on sand for improved traction. Sand crystals increase friction and help prevent vehicle tires from slipping on slick roads, making travel easier for drivers.

Sand can also help prevent new ice from forming on roads. Imagine each grain of sand as a tiny little ball. In each scoop of sand, there are millions of balls constantly in motion. Each time a car drives over them or the wind blows, the grains move around.

The movement of the sand grains makes it difficult for water molecules to stick together and form ice. Although sand may not entirely prevent ice from forming, its constant motion makes it difficult for ice to build up as quickly as it can on untreated roads.

Did you get it?

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Try It Out

Brrr! We're shivering from this Wonder of the Day! Be sure to check out the following activities with a friend or family member:

Want to conduct a really simple, really cool experiment? Take an ice cube from your freezer and place it on a plate. Sprinkle a pinch of salt over one half of the ice cube. Put the plate with the ice cube in the refrigerator. After about 10 minutes, take the ice cube out and observe what has happened. What do you notice about the salted side?

If you're up for some more advanced experimenting, try measuring the effect salt has on the freezing temperature of water. Place water in a plastic cup and put it in the freezer, when it begins to become slushy, use a thermometer to measure the temperature of the water. Grab another plastic cup and fill it with water and a teaspoon of salt. Place the salt water into the freezer and measure the temperature when it begins to get slushy. How does the freezing point of your salt water compare to plain water?

Putting salt and sand on the roads can help prevent accidents in snowy and icy weather. But what else can drivers do? Talk with an adult friend or family member about driving in snowy and icy conditions. How do they drive differently when the weather is bad? What extra precautions do they take?

Nancy Bonilla

Wonderopolis

Feb 26, 2016

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"Why Is Sand or Salt Spread On Roads When It Snows?" Wonderopolis. Web. 26 February 2016.

larry

Wonderopolis

Wonderopolis

Jan 21, 2016

Hi, Yoseph! We're glad this Wonder was helpful! It is a mixture of sand and salt. To learn more about the exact measurements of each ingredient, we encourage you to keep researching. Thanks for WONDERing with us! :)

Juju

Wonderopolis

Dec 3, 2015

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Wonderopolis

Nov 9, 2015

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Wonderopolis

Oct 5, 2015

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Cece

Wonderopolis

Oct 1, 2015

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Wonderopolis

I think that is super cool topic! I didn't really know sand cleans and melts the snow! Every Christmas when it snows, me and my dad go outside to the driveway and we pour salt on it so we can drive in that space. Also, he told me that it is not safe to drive to places when the roads are icy. Are there any more minerals that melt ice???

Wonderopolis

Nov 13, 2011

That's a GREAT question, Alexandra! We'll have to do some more WONDERing to find out if there are other minerals that melt ice! Thank you for sharing your personal connection to this Wonder, too! We're glad you visited Wonderopolis today! :-)